The present invention relates to the art of improving octane rating of premium gasolines by providing a process for preparing an additive by alkylating an isoparaffin with olefin.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 -C.sub.5 olefin with isobutane in the presence of an acidic catalyst, producing the so-called alkylate. This is a very valuable ingredient in the manufacture of premium gasolines because of its high octane rating and good response to tetraethyl lead.
Traditionally, processes in the industry include the use of hydrofluoric acid or sulfuric acid and a catalysis carried out under controlled temperature conditions. Low temperatures are required in the sulfuric acid process to minimize the side reaction of olefin polymerization, and the acid strength has to be maintained at 98-100 percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature-sensitive and the acid is easily recovered and purified. However, any traces of water in the feedstock must be eliminated owing to the extreme corrosivity of hydrofluoric acid/water solutions.
Although alkylation processes using liquid, acidic catalysts are commercially successful, inherent disadvantages arise, in addition to those mentioned above, in the use of such catalysts including handling and disposal of corrosive materials.
Consequently, substantial efforts have been made to develop a viable isoparaffin-olefin alkylation process using a solid catalyst which is commercially acceptable. For example, U.S. Pat. No. 3,251,902 shows the use of an ion-exchanged crystalline aluminosilicate having a reduced number of available acid sites in fixed bed, liquid phase alkylation of C.sub.4 -C.sub.20 branched-chain paraffins with C.sub.2 -C.sub.12 olefins, and teaches that the C.sub.4 -C.sub.20 branched-chain paraffin should be allowed to substantially saturate the crystalline aluminosilicate before the olefin is introduced to the alkylation reactor. U.S. Pat. No. 3,450,644 discloses a method for regenerating a zeolite catalyst used in hydrocarbon conversion processes involving carbonium ion intermediates. U.S. Pat. No. 3,541,180 discloses a method for improvement of thermal alkylation processes for reacting isobutane with ethylene or propylene which involves using solid sodalite or ultramarine catalysts. U.S. Pat. No. 3,549,557 shows alkylation of isobutane with C.sub.2 -C.sub.3 olefins using certain crystalline aluminosilicate zeolite catalysts in fixed bet, moving bed or fluidized bed systems, with olefin being preferably injected at various points in a fixed bed system. U.S. Pat. No. 3,644,565 discloses alkylation of a paraffin with an olefin in the presence of a catalyst comprising a Group VIII noble metal distended on a crystalline aluminosilicate zeolite, where the catalyst is pretreated with hydrogen to promote selectivity. U.S. Pat. No. 3,647,916 teaches an isoparaffin-olefin alkylation process using an ion-exchanged crystalline aluminosilicate, which includes alkylating at isoparaffin/olefin molar ratios below 3:1 and regenerating the catalyst. U.S. Pat. No. 3,655,813 discloses a process for alkylating C.sub.4 -C.sub.5 isoparaffins with C.sub.3 -C.sub.9 olefins using a crystalline aluminosilicate zeolite catalyst, wherein a halide adjuvant is employed in the alkylation reactor, isoparaffin and olefin are introduced into the alkylation reactor at specified concentrations, and catalyst is continuously regenerated outside the alkylation reactor. U.S. Pat. No. 3,706,814 discloses an isoparaffin-olefin alkylation process, using a crystalline aluminosilicate zeolite catalyst, which includes addition of C.sub.5 + paraffins "such as Udex raffinate" or C.sub.5 + olefins to the hydrocarbon feed to the alkylation reactor, and also involves the use of specific reactant proportions, halide adjuvants, etc. Various solid catalysts useful in isoparaffin-olefin alkylation have been disclosed in the above-listed patents and also, for example, in the following. U.S. Pat. No. 3,236,761 discloses the use, in alkylation, of crystalline aluminosilicate zeolites having silica to alumina mole ratios above 3 and also discloses the use of various metals exchanged and/or impregnated on such zeolites. U.S. Pat. No. 3,624,173 discloses the use, in isoparaffin-olefin alkylation, of crystalline aluminosilicate zeolites containing gadolinium. U.S. Pat. No. 3,738,977 discloses alkylation of paraffins with ethylene using a crystalline aluminosilicate with a Group VIII metal distended thereon in which the catalyst has been pretreated with hydrogen. U.S. Pat. No. 3,917,738 shows a process for alkylating an isoparaffin with an olefin using a solid, particulate catalyst capable of absorbing the olefin. The isoparaffin and the olefin are admixed to form a reactant stream in contact with catalyst particles at the upstream end of an adsorption zone after which the reactants are passed concurrently with the catalyst so that a controlled amount of olefin is adsorbed onto the catalyst before the combination of reactants and catalyst is introduced into an alkylation zone. This controlled olefin adsorption prevents polymerization of the olefin during alkylation.
Problems arise, however, in the use of solid catalysts in that they appear to age rapidly and cannot perform effectively at high olefin space velocities.